The present invention relates to an engine air-fuel ratio control apparatus for controlling a fuel injection amount so that an air-fuel ratio of an air-fuel mixture to be supplied to an internal combustion engine becomes equal to a theoretical air-fuel ratio.
Under the so-called modern control theory, such an air-fuel ratio control apparatus is arranged to construct a dynamic model of a system for controling the air-fuel ratio in an engine by approximating an auto-regressive model whose model order is 1 and includes a dead-time P (P=0, 1, 2, . . . ) concurrently with taking into account the disturbance, thereby determining an air-fuel ratio control amount in accordance with a state variable quantity and an optimal feedback gain predetermined on the basis of the constructed dynamic model. The optimal feedback gain is determined so that responsiveness and stability are compatible with each other in various operating conditions as disclosed in the Japanese Patent Provisional Publication No. 1-110853. There is a problem which arises with the air-fuel ratio control apparatus based upon the modern control theory, however, in that at the time of speed-reduction which causes the intake pipe pressure to considerably lower, the combustion becomes unstable due to decrease in the flaming speed so as to take a slight misfire state whereby the air-fuel ratio varies. In this case, the air-fuel ratio control apparatus tends to be quickly responsive to the air-fuel ratio variation, whereby the air-fuel ratio correction coefficient FAF greatly varies to result in promoting the air-fuel ratio variation to deteriorate the controllability at the time of the speed-reduction (see FIG. 6).